In order to analyze the atomization mechanism of gas liquid swirl injector, a temporal linear stability analysis method was used to investigate the conical liquid film with coaxial annular gas flow. The dispersion equation of a conical liquid film with coaxial annular gas flow was derived and a prediction model was proposed to calculate the parameters needed in solving the dispersion equation. The results show that the film thickness at the injector exit decreases with the increase of pressure drop while the spray cone angle, liquid film velocity and liquid film axial velocity increase with the increase of pressure drop. The sinuous mode disturbance wave dominates the breakup process of conical liquid film with coaxial annular gas flow because the growth rate of sinuous wave is much larger than that of varicose wave. When the gas flow velocity is small, the increase of gas velocity reduces the relative velocity of gas and liquid, which weakens the gas liquid interaction and eliminates the growth rate and frequency of the dominant wave. Finally the breakup time and breakup length increases. However, when the gas velocity is larger than a critical value, the growth rate and frequency of the dominant surface wave increase rapidly with the increase of gas flow velocity, which in turn lowers the breakup time and breakup length immediately.